With increasing wind power, interaction of the wind turbine generators (WTGs), e.g., doubly-fed induction generators (DFIGs), may cause oscillation in the wind farm and endanger the power systems, so it is necessary to optimize the parameters of the WTG to mitigate the interaction. The existing study often focuses on the interaction between the wind farm and the system, but ignores the interaction among the WTGs. In this paper, the interaction path and parameter optimization for the wind farm with the DFIGs is studied. The novelties are, 1) The analytical expression for interaction path in the wind farm is newly derived with the proposed expanded effective open-loop process (EEOP), 2) a new indicator is proposed to evaluate the gain of the interaction paths for two-input, two-output characteristics of the DFIGs, 3) analytical expression for the sensitivities of the interaction with respect to the control parameters is newly derived, and 4) a multi-objective model including the sensitivity model to optimize the parameters of the DFIGs is proposed to mitigate the interaction. In the numerical analysis, a wind farm with multi-DFIG is used to verify the interaction paths and the indicator, and validate the mitigation effect with the optimized parameters of the DFIGs.

The increasing wind power decreases power system damping and may intensify low-frequency oscillation (LFO). The LFO are usually damped by the power system stabilizer (PSS) at synchronous generator (SG), and now by the power oscillation damper (POD) at doubly-fed induction generator (DFIG). The existing damping torque analysis (DTA) sets the parameters of the PSS and evaluates its damping capability, but can not be applied to the POD due to the difficulty of finding the damping path related to the DFIG and the coupling between the POD and the DFIG, which are studied in this paper. At first, the analytical expression of the coupling between the POD and DFIG is newly derived with linear fractional transformation (LFT) technique. Then the path damping torque analysis (PDTA) is proposed to reconstruct the damping path of the POD. Thirdly, the damping indicator based on the return difference matrix is proposed to evaluate the contribution of damping path to the LFO. Finally, numerical results of test system are given to validate effectiveness and accuracy of the proposed model, and parameter optimization to the multi-input POD (MIPOD) is performed to show the application value of the proposed model.

Low-frequency oscillation (LFO) of the synchronous generators in power systems by wind power is boring. To improve the robustness of the damping control scheme, this paper applies the sliding mode control (SMC) at the doubly fed induction generator (DFIG), with the parameter of the SMC optimized by the eigen-sensitivity. The originalities lie in, (1) the states strongly associated with the critical modes are newly applied to design the sliding surface, (2) the closed-loop model of the power system with the improved equivalent control is derived to analyze the damping effect on the critical modes and the undesirable effect on the noncritical modes, (3) the gain in the improved equivalent control is optimized to damp the critical and noncritical modes, and (4) the eigenvector sensitivity is improved to derive the second-order eigen- sensitivity to solve the nonlinear optimization. Numerical results show that the proposed model damps the critical modes effectively for different wind speeds, while the undesirable effect on the noncritical modes is avoided.

Wind turbine generators, e.g., doubly-fed induction generators (DFIGs), have little inertia, thus reduce power system stability and intensify low-frequency oscillation (LFO). The LFO previously damped by the synchronous generators now may be damped by power oscillation damper (POD) at the DFIGs. The existing POD studies are for the aggregated wind farm, ignoring the difference among the DFIGs, and the POD placement at all DFIGs may be unnecessary. In this paper, the optimal placement of the PODs in wind farm to enhance the stability margin and mitigate the LFOs of power system is studied, with the following novelties, 1) The feedback loops for multi-DFIG wind farm and PODs are newly derived to show the effect of PODs on stability margin. 2) With the perturbation analysis to the open-loop gain function, the tight bound to describe the effect of POD placement on stability margin is proposed. 3) An optimization model with the bound of stability margin is proposed to place the PODs, and solved with the improved simulated annealing algorithms using the greedy search. Numerical analysis verifies the correctness of the proposed stability margin bound and the effectiveness of the optimal placement to PODs.